Use of electrically controllable variable inductor for tuning purposes



May 24, 1960 s. H. DE wrrz 2,938,130

USE OF ELECTRICALLY CONTROLLABLE VARIABLE INDUCTOR FOR TUNING PURPOSES Original Filed Oct. 25, 1956 FIG. F/a. 2

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USE OF ELECTRICALLY CONTROLLABLE VARI- ABLE INDUCTOR FOR TUNING PURPOSES Original application Oct. 25, 1956, Ser. No. 618,411, now

Patent No. 2,882,403, dated Apr. 14, 1959. Divided and this application May 20, 1958, Ser. No. 738,908

2 Claims. (Cl. 33378) This invention relates to electrically-controllable variable inductors, and particularly to the use of such inductors for controlling the tuning of electrical circuits, such as resonant circuits, selective networks, oscillators, communication receivers and the like.

This application is a true division of application Serial No. 618,411, filed October 25, 1956, which issued as Patent No. 2,882,403 on April 14, 1959, to Gerhard H. De Witz.

One of the difliculties experienced in the practical application of the usual inductor of this type is that its obtainable Q, i.e., ratio of inductive reactance to resistance, is limited at high frequencies, whereas many electrical circuits in which such inductors might be used for tuning purposes, such as oscillators and communication receivers, require a relatively high value of Q for operation at such high frequencies. For example, to obtain a high degree of selectivity in a high frequency communication receiver, it is desirable to use high Q tuned circuits therein. Of course it is possible to obtain the same degree of selectivity by the use of capacitively tuned circuits or a greater number of cascaded, inductively-tuned stages of lower Q values, but these alternatives are relatively uneconomical.

A general object of the invention is to improve electrically-controllable variable inductors so as to make them suitable for tuning electrical apparatus requiring relatively high values of Q" for proper operation at certain frequencies.

A related object is to improve electrically-controllable variable inductors so as to make them suitable for controlling simultaneously the tuning of two or more coupled resonant circuits.

Another object is to provide a high degree of selectivity in an electrical circuit with a small number of inductively-tuned tuning stages.

Another object is to control inductively the tuning of and the coupling between electrical resonant circuits. A more specific object is to control inductively the frequency of oscillation of an oscillator, for example, an oscillator of the Wien-bridge type, efficiently and economically.

The circuits in accordance with the invention for accomplishing these objects utilize an electrically-controllable variable inductor which is a modification of one disclosed in the US. patent to L. M. Carver, No. 2,708,- 210, issued May 10, 1955. This modified inductor consists ofv a magnetic core in the form of a toroid or ring with a single control winding and a plurality of controlled windings wound. on this core. The magnetic core is formed from a ferromagnetic material suitable for high frequencies and of high permeability, and includes a slot or window through thesides of the core at two or more spaced points around its periphery. Each of the controlled windings extends through a different one of the slots and is wound around the core portions on each side of that slot in such manner that any magnetic fields United States Patent 2,938,180 Patented May 24, 1960 established in the core in response to current flowing through the winding will close around the respective slot, with the result that electromagnetic couplings between the controlled windings and between each of these windings and the control winding will be negligible. The inductances of the controlled windings vary in accordance with the amount of saturation produced in the magnetic core by the control winding which in turn is dependent on the amplitude of the control or saturating D.-C. current applied to the latter winding.

In accordance with one modification of the invention, the controlled windings of the above-described variable inductor are associated with each other and with auxiliary fixed value capacitors or such capacitors and one or more fixed value inductors to form coupled resonant circuits, the tuning of which are simultaneously controlled inductively by controlling the amplitude of D.-C. current applied to the control winding. In one type of circuit so formed, the connections are such that the coupling between the resonant circuits is constant and independent of frequency. In another type of circuit so formed, the connections are such'that the couplings be tween the resonant circuits vary with frequency.

A feature of the invention is the use of the abovedescribed variable inductor to tune an oscillator of the Wien-bridge type to the desired oscillation frequency by balancing the Wien-bridge located in an oscillator negative feedback path, at that frequency so as to reduce the negative feedback to zero.

The various features and objects of the invention will be better understood from the following detailed description thereof when it is read in conjunction with the various figures of the drawing in which:

Fig. 1 shows a perspective view of one embodiment of the electrically-controllable variable inductor used in the circuits of the invention;

Figs. 2 and 3 show schematically two of the many possible connections in accordance with the invention of the windings of the variable inductor of Fig. l to auxiliary capacitors or such capacitors and fixed value inductors to provide electrically-controllable, inductively-tunable coupled resonant circuits;

Fig. 4 shows diagrammatically the location and direction of the magnetic fields established in the magnetic core of the variable inductor of Fig. l by current flowing in the controlled windings, for a preferred arrangement and relative poling of these windings; and

Fig. 5 shows schematically the application of the variable inductor of Fig. 1 to control the oscillation frequency of an oscillator of the Wien-bridge type, in accordance with the invention.

As shown in Fig. 1, the variable inductor 10 used for tuning purposes in the circuits of the invention includes a magnetic core 11 in the form of a toroid or ring; and a single control winding 12 having terminals 13, and two controlled signal windings 14 and 15, having terminals 16 and 17, respectively, wound on the core 11. The magnetic core 11 may consist of any suitable magnetic material having a high permeability, for example a ferromagnetic material or a ferromagnetic ceramic material commonly referred to as ferrite and described in the US. Patents 2,452,529; 2,452,530 and 2,452,531 to Snoeck, in finely divided or laminated form.

The core 11 has two rectangular windows 18, 19 formed in its opposite sides. Although it is usually preferable for reasons of symmetry and balance that these slots in the core be located at diametrically opposite points, other relative locations may prove move suitable for particular applications of the variable inductor.

Each of the controlled signal windings 14 and 15 has two equal series-connected sections. The winding 14 ex:

tends through the core slot "18 with one of its sections wound around the adjacent portion of the core on one side of that slot, and the other of its sections wound around the adjacent core portion on the opposite side of that slot. Similarly, the winding 15 extends through the other core slot 19 with one of its sections wound around the adjacent core portion on one side of that slot and the other of its sections wound around the adjacent core portion on the opposite side of that slot.

The winding arrangements of the controlled signal windings 14 and 15 above described are such that the flux lines of the magnetic fields established in the core 11 due to current flowing in these windings will form closed loops around the respective slots 18 and 19. Thus, the electromagnetic mutual couplings between the two signal windings 13 and 19 and between each of these windings and the control winding 12 will be negligible. This decoupling efi'ect may be increased by relatively poling the two signal windings 14 and 15 so that the fluxes due to the currents flowing in their windings pass around the slots 18 and 19 in the core 11 in reversed senses, as indicated by the arrows in Fig. 4. Due to the extreme shortness of the flux paths around the slots 18 and 19 in the core, relatively high Q values can be obtained with this variable inductor making it particularly suitable for use in electrical circuits requiring high Q values for proper operation at high frequencies.

The inductances of the two controlled signal windings and 15, and thus their inductive effects in any electrical circuit in which they maybe connected through their terminals 16 and 17, respectively, may be varied by varying the amount of saturation in the core. This is done by varying the amplitude of saturating D.-C. current supplied to the control winding 12 from a suitable source connected across the terminals of that winding. In order to obtain the maximum inductive effect for a given amount of applied D.-C. control current, the control winding 12 preferably is wound so that it surrounds substantially all of the core 11 except for the regions thereof oc cupied by the slots 18 and 19 and the signal windings 14 and 15. For reasons of clarity the control winding 12 is shown in Fig. 1 as having only relatively few turns, but in practice it would usually have a large number of turns.

Fig. 2 shows one of the many electrical circuit arrangements in which a variable inductor such as shown in Fig. 1 could be used for tuning and coupling purposes. In this arrangement a capacitor 20 is connected across the terminals 16 of the controlled winding 14 to form therewith one resonant circuit, a second capacitor 21 is connected across the terminals 17 of the winding 15 to form therewith a second resonant circuit, and the two resonant circuits are coupled by a direct connection 22 between corresponding points on the two windings 14 and 15, as shown. The inductances of the windings 14 and 15 in the respective resonant circuits are simultaneously adjusted in equal amounts to the values required to tune the resonant circuits to the desired frequencies by the application of D.-C. control current of proper value to the control winding 12 to provide the necessary amount of saturation in the magnetic core on which the windings 12, 14 and 15 are wound in the manner previously described in connection with Fig. 1. Because of the direct electrical connection 22 between corresponding points on the two controlled windings in the two resonat circuits, the coupling between these circuits will be constant regardless of the frequency to which they are tuned.

Although not so shown in Fig. 2 of the drawings, the connection 22 between the controlled windings 14 and 15 in some applications alternatively might include circuit elements such as capacitors or resistors of suitable values, hereby achieving a coupling of any desired relationship to frequency. As the Q of the circuit arrangement may increase or decrease with the amount of saturation of the core, it is usually desirable "to obtain constant bandwidth.

4 Constant bandwidth between double tuned circuits is obtained by keeping the factor KQ constant, wherein K represents the amount of coupling; therefore, as Q varies, K should vary inversely. Additional circuit elements in the coupling connection 22 for this purpose, therefore, may be desirable.

The circuit arrangement of Fig. 3 differs from that of Fig. 2 essentially in the inclusion of an inductor 23 of fixed value in series with the capacitor in each resonant circuit, and common to both resonant circuits so as to provide a coupling between these circuits which varies with frequency.

Fig. 5 shows how a variable inductor such as shown in Fig. 1 may be utilized for controlling the frequency of oscillation of an oscillator of a modified Wien-bridge type. As shown, the oscillation portion proper of this oscillator comprises an amplifier represented by the box, and a positive feedback circuit connecting its output to its input, represented by the box 25. This amplifier-positive feedback combination 24, 25 by itself will oscillate at some frequency determined by the characteristics of its component circuit elements. The output of the amplifier 24 is connected directly to the control grid-cathode circuit of a three-electrodevacuum tube 26, operating as an amplifier and inverter, the anode-cathode circuit of this tube including a suitable load resistor 27, a D.-C. voltage source for positively biasing the anode with respect to the oathode, and a resistor 28 connected between its cathode and ground. The anode of the tube 26 is connected through a capacitor 29 and a modified Wien-bridge including the controlled windings 14 and 15 of a variable inductor such as shown in Fig. 1, in series to the cathode of that tube. In the particular modified Wien-bridge shown, the winding 14 and resistor 30 in series therewith are included in one bridge arm and the Winding 15 and the resistor 31 in shunt therewith in another bridge arm. The bridge output is connected to the input of the amplifier 24 in such manner as to provide a negative feedback connection thereto. The negative feedback provided by the connections described from one output of the amplifier 24 to its input will exist at all frequencies except the balance frequency of the bridge (that of zero output from the bridge), and by suitable selection of circuit constants for the negative feedback connection, the amount of negative feedback may be made sufficient to overcome the positive feedback provided by connection 25 and prevent oscillation of the amplifier 24 except at this balance frequency.

The control or saturating winding 12 of the variable inductor of Fig. 1 and the associated adjustable amplitude D.-C. current supply circuit are utilized in the oscillator of Fig. 5 to vary the saturation of the magnetic core l l (shown diagrammatically) of the variable inductor and thus to vary simultaneously in equal amounts the inductance values of the two controlled signal windings 14 and 15 of the inductor in the manner previously described in connection with Fig. 1, to maintain the bridge balanced at a frequency determined by the adjusted inductance values of these windings. By suitable selection of the amplitude of the DC. control current supplied to the control winding 12, the adjusted inductance value of the controlled windings 14 and 15 in the Wienbridge may be made such that the bridge is balanced at the desired frequency of oscillation of the oscillator. When the bridge is balanced for this frequency, there will be zero output from the bridge and no negative feedback voltages will be applied to the input of the amplifier-positive feedback combination 24, 25. This combination, therefore, is constrained to oscillate at this frequency. This frequency is determined by the inductances of the controlled windings of the electrically controllable variable inductor since, in this configuration, the frequency of oscillation is inversely proportional to the inductance rather than to the square root of the inductance.

As is well known, the bridge circuit of an oscillator of the Wien-bridge type allows a voltage of only one frequency to be effective in the circuit because of the degeneration and place shift provided by the circuit. In the oscillator circuit of Fig. 5, oscillation can only take place at the frequency f which permits the voltage across resistor 3 1 (in shunt with winding 15), which is the input signal to the amplifier 24, to be in phase with the output voltage of the amplifying tube 26, and for which the positive feedback voltage (that provided by circuit 25) exceeds the negative feedback voltage. A voltage of any other frequency will cause a phase shift between the output of amplifier 26 and the input to amplifier 24, or will be attenuated by the high degeneration of the circuit, so that the feedback voltage is not adequate to maintain oscillation at a frequency other than i Although the electrically controllable variable inductor in the circuits of the invention as shown in Figs. 1 and 4 and described above has only one pair of slots in the magnetic core, it is to be understood that in circuits utilizing larger toroids or rings for the core, three slots 120 angular degrees apart or four 90 degrees apart may be used. Where more than two slots are used, there may be a small amount of undesired coupling. This may be easily minimized by any suitable means for preventing coupling back from a high to a low level stage, and by orienting the slots to reduce the flux linkages between windings to a minimum. Other changes in the variable inductor structures or the circuits in which they are used which are within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In combination with electrical apparatus including at least one pair of electromagmetically coupled resonant circuits for determining its operating frequency, means for simultaneously, controlling the tuning of and coupling between said resonant circuits comprising variable inductor means including a high permeability saturable magnetic core in the form of a toroid and having a plurality of slots through its sides, respectively located at different spaced points around its periphery, a control winding and a plurality of controlled windings respectively wound on different portions of said core, each of said controlled windings extending through a different one of said slots and wound around the portions of said core on opposite sides of that slot in such manner that the flux lines of any magnetic fields established in said core due to current flowing in that controlled winding will form a closed loop respective slot in reversed sense with respect to the flux lines around the oppositely disposed slot in the core thereby making the electromagnetic mutual couplings between the controlled windings and between each of these windings and said control winding substantially negligible, each of said controlled windings being connected in a different one of said resonant circuits so that its inductance value determines the resonance frequency of that circuit, an electrical connection other than said core between the controlled windings in each pair of coupled resonant circuits, to provide a coupling between these circuits of a desired relationship with respect to their resonant frequencies, the inductance value of each of said controlled windings being variable in accordance with the amount of saturation produced in said core, a source of D.-C. control current of adjustable amplitude connected to the control winding on said core, said control winding operating to saturate the core in an amount dependent on the amplitude values of the control current applied to that winding from said source, which is selected such as to control the inductance of each of said controlled windings and thus the resonance frequency of each of said resonant circuits to make the operating frequency of said apparatus of the desired value.

2. In combination with electrical apparatus requiring a high ratio of inductive reactance to resistance for proper operation and including electromagnetically coupled resonant circuits having one or more capacitors therein for determining its operating frequency, means for simultaneously controlling the tuning of and electromagnetic coupling between said resonant circuits to provide the desired operating frequency for said apparatus comprising a variable inductor having a toroidal core of laminated ferromagnetic material of high permeability, said core having a plurality of slots through its sides at respectively different spaced points around its periphery, a plurality of equal controlled windings each having two equal seriesconnected sections, and a single control winding wound on said core, each of said controlled windings extending through a different one of said slots with one section thereof wound around the adjacent core portion on one side of the slot and the other section thereof wound around the adjacent core portion on the opposite side of said slot in such manner that the flux lines of any magnetic fields established in said core due to current flow in that controlled winding will close around the respective slot and in reversed sense with respect to the flux lines around the oppositely disposed slot in said core and said control winding surrounds substantially all of said core except for the portions occupied by said slots and said controlled windings in order to provide the maximum inductive effect for a given amount of control current applied to the control winding, this winding arrangement of said controlled and control windings making the electromagnetic mutual couplings between the controlled windings and between each of these windings and said control winding negligible, a different one of said controlled windings being connected in circuit with a capacitor in each of said resonant circuits so that its inductance value determines, the resonance frequency of that resonant circuit, a source of D.-C. control current adjustable in amplitude, connected to said control winding, said core being saturated in accordance with the amplitude of the control current applied to the control winding from said source, the inductance of each controlled winding being variable in accordance with the amount of saturation of said core, said variable inductor enabling proper operation of said apparatus with a relatively small number of coupled resonant circuits because of the high ratio of inductance reactance to resistance attainable due to the extremely short length of the magnetic flux paths in said core.

References Cited in the file of this patent UNITED STATES PATENTS 1,853,969 Ganz Apr. 12, 1932 2,051,364 Braden Aug. 18, 1936 2,052,703 Farnham Sept. 1, 1936 2,055,996 Braden Sept. 29, 1936 2,569,309 Hepp Sept. 25, 1951 2,661,459 Schmidt Dec. 1, 1953 2,708,219 Carver May 10, 1955 2,803,804 Kihn Aug. 20, 1957 2,820,109 De Witz Ian. 14. 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2 938, 180 May 24 1960 Gerhard Ha De Witz It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5 line 49 before "'respective" insert around the -3 column 6, line 51 for "inductance" read inductive Signed and sealed this 29th day of November 1960.,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Ofi'icer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 2 938 180 May 24 1960 Gerhard HG De Witz It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5 line 4L9 before "respective" insert around the --3 column 6 line 51 for "inductance" read induc tive Signed and sealed this 29th day of November 1960o SEA L) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

